Liquid crystal displays (LCDs) are commonly used in various electronics, such as cell phones, laptops, electronic tablets, televisions, and computer monitors. However, conventional LCDs can be limited, as compared to other display devices, in terms of brightness, contrast ratio, efficiency, and/or viewing angle. For instance, to compete with organic light emitting diode (OLED) technology, there is a demand for higher contrast ratio, color gamut, and brightness in conventional LCDs while also balancing product cost and power requirements, e.g., in the case of handheld devices.
The display properties of LCDs can be limited by various key components of the LCD panel, e.g., the backlight and the color filter. For example, conventional LCDs can comprise a backlight utilizing white light emitting diodes (LEDs) and patterned transmissive color filters to define red, green, and blue pixels. Blue light emitted by LEDs can be converted to white light, which can then be transmitted through two polarizers and an LCD stack, and subsequently filtered through a color filter element. During this conversion/filtering process, a large portion of the light produced by the LED can be lost such that as little as 10% of the light generated by the backlight can be emitted by the display. Moreover, the power efficiency of such LCD devices can be very low, with an overall electrical to optical power efficiency of less than about 1%. Finally, the color gamut of the LCDs can be limited, for example, the color gamut can be as low as 70% of the standard set by the National Television System Committee (NTSC), due to non-optimal spectral content of white LEDs in combination with reduced transmission of the color filter element.
Recent advances in LCD technology suggest that the efficiency and color gamut of LCDs can be improved by using emissive (e.g., photoluminescent) color filters instead of transmissive color filters. For example, cold cathode fluorescent lamps (CCFLs) have been used in combination with red, green, and blue phosphor materials, which can be placed as desired in the respective pixel areas of a photoluminescent color filter. Blue LED backlights are also available, in which case red and green phosphors can be used, and the areas of the color filter corresponding to blue pixels could be left transparent, or provided with light scattering features.
Quantum dots (QDs) have also emerged as an alternative to conventional phosphors and can, in some instances, provide improved precision and/or narrower emission lines, which can improve, e.g., the LCD color gamut. In addition to high internal quantum efficiency and low quantum defect (the wavelength difference between excitation and emission light), QD materials can provide relatively high color purity, e.g., they can emit light in a relatively narrow spectral band as compared to conventional phosphors, and the central wavelength of that band can be tuned relatively easily by changing the size of the individual QDs. However, devices patterned with QDs can be expensive and/or complex to produce.
Traditional color filters can be produced using separate steps for the red, green, and blue pixel areas, each step comprising depositing the corresponding phosphor in the corresponding pixel area. For example, a continuous layer of material can be deposited and then patterned using photolithography, lift-off or etching, or can be deposited through a photolithographically defined shadow mask. This technology can, however, be complex and expensive, and may not be easily adaptable to QD materials. QD materials, for example, can be too heavy to be deposited by thermal evaporation and, thus, have instead been deposited using dip-coating or slot-coating. Newly developed processes include micro-contact printing and inkjet printing, but these processes are not free of their own challenges. Micro-contact printing can result in material waste and may be difficult to scale up. Inkjet printing, on the other hand, may present challenges in terms of developing suitable solution chemistry for precise pixel definition and/or preserving photoluminescent efficiency after drying. Further, regardless of the particular deposition process, QD films can also have reduced light extraction efficiency, e.g., for a flat surface, less than about 20% of the light generated by the QDs can be emitted by the film due to total internal reflection (TIR).
Accordingly, it would be advantageous to provide light-emitting devices, e.g., for LCDs, which can exhibit improved power and optical efficiency, while also reducing material waste, thereby lowering the cost of such devices, and/or simplifying the manufacturing process, thereby reducing production time.